Generally, these known devices are mounted on (or made integral with) a bedplate which closes the front side of the freezing cylinder of an ice-cream making machine, and comprise a cylinder member communicating, in proximity of its lower end, via a radial duct extending through said baseplate, with the freezing chamber. The said cylindrical member is open at both ends and is formed at its bottom with a suitably shaped dispensing hole. A piston, slidably fitted in said cylindrical member, is reciprocable through suitable control means (usually, a lever pivotably secured by one end to the upper region of said cylindrical member), from a first or lowered position wherein the lower end of said piston closes both said communication duct and said dispensing hole in the bottom of the cylindrical member, to a second or raised position wherein the lower end of said piston is raised to a level above said communication duct so as to allow the free flow of ice-cream from the freezing chamber outwards through the dispensing hole in the bottom of the said cylindrical member, or cylinder.
The piston-cock dispensing devices of the type mentioned above are classified either as simple dispensers, wherein one piston-cock closes and controls the outfow from one single freezing cylinder of the machine, and multiple dispensers, or "mixers" wherein between each pair of dispensing cocks controlling the outflow from two side-by-side cylinders of one ice-cream making machine, a third dispensing cock is interposed communicating at its bottom with both cylinders of the ice-cream making machine in order to dispense a mixed-flavor ice-cream through its dispensing hole.
Dispensing piston-cocks for ice-cream machines, made entirely of metal, are known, for example, from U.S. Pat. No. 2,737,024. However, such a dispensing cock (comprising a closure plate, a cylinder and a piston) made entirely of metal involves a number of disadvantages, one of which being that, due to the high thermal conductivity of metal, an excessive transfer of cold to the entire dispensing cock takes place, with resulting thermal losses outwards and formation of condensate and frost on said dispensing cock.
In order to overcome the above and other disadvantages of the above mentioned prior art ice-cream dispensing cocks, it has been proposed, as described in the Italian Pat. Nos. 599,450 and 621,459 to Poerio Carpigiani (corresponding to U.S. Pat. No. 3,052,381) to make such dispensing devices (baseplate, cylinder or cylinders, and piston or pistons) entirely of plastics material. Thus, thanks to the insulating characteristics of plastics, this dispensing device avoids, inter alia the thermal loss outwards from the freezing chamber and the formation of frost or of frozen condensate on the dispensing device and parts associated therewith.
This construction has involved a real technological revolution in the field of ice-cream making machines and has been fully successful, whereby it may be stated that all dispensing devices of the kind set forth above are presently made of plastics, in accordance with the teachings of the above mentioned patents to Carpigiani.
Upon the development of the latest ice-cream making machines, for example the machine described in the Italian Patent Application Ser. No. 12451 A/81, or in the U.S. Pat. No. 4,476,146 to Ezio Manfroni, wherein a pasteurizing proper of the ice-cream mix and of the frozen product therein is performed intermittently in the ice-cream mix feeding tank and in the freezing cylinder of the machine by heating them up to pasteurizing temperature and subsequent cooling, in order to prevent the formation, multiplication and growth of pathogenetic germs in the ice-cream machine, the problem has arisen of the simultaneous sterilization of the ice-cream dispensing elements which are associated with the freezing chamber of the said machines.
In fact, the above mentioned prior-art dispensing devices made of plastics material, just due to their thermal insulating characteristics that have caused their full success with the traditional ice-cream making machines, hindered the transfer of heat to the area of the device between the lower end of the dispensing cylinder and the lower end of the dispensing piston, whereat a thin layer of ice-cream is usually stagnant, thus hindering a thorough sterilisation of all the portions of the machine contacted by the flow of ice-cream.